1. Field of the Invention
The present invention relates to an improved process for removing pollutants, especially sulfur dioxide and nitrogen oxides, from effluent gases resulting from the combustion of carbonaceous fuels. Specifically, the invention relates to a synergistic combination of two processes which provide for the removal of sulfur dioxide from high sulfur content coals (over 2% sulfur).
2. Description of the Prior Art
In modern industrial society an extensive amount of effluent gas is generated from automobiles and industrial plants. The adverse impact on human health of certain of the pollutants has only relatively recently begun to be appreciated. Specifically, sulfur dioxide (SO.sub.2) and nitrogen oxides of various forms (NO.sub.x) are believed to be especially objectionable. Although particulates such as ash and dust are also deleterious to the human body, these solid pollutants are usually easier to remove than the sulfur dioxide and nitrogen oxides.
The pollution problem of recent years has been exacerbated by the energy crisis which has effected most industrial societies including the United States. Specifically, increasing our energy output has become an important national priority. Accordingly, great pressure has developed to use the high sulfur coal which the United States has in relative abundance, and thereby lessen our dependence on expensive imported oil. However, use of the high sulfur content coal causes an increase in the air pollution levels and, specifically, the amount of sulfur dioxide in the air. This poses a dilemma of choosing between using high sulfur coal along with the resulting adverse health effects of high sulfur dioxide pollution or exacerbating the energy shortage by not using the high sulfur coal.
In order to avoid the dilemma posed by the above choice between energy or clean air, numerous methods and apparatuses have been developed to minimize the sulfur dioxide pollution caused by the burning of high sulfur coal. Typically, these prior methods and apparatuses are costly and have large operating and maintenance problems. Often these techniques are constrained by various considerations which render them useful only in very limited conditions.
The prior art has included various wet scrubbing or washing processes. These processes use an aqueous alkali solution which is sprayed into the effluent gas as it passes through a chamber or tower. Relatively large quantities of water are used in the injected slurry in order to saturate the effluent gases. In addition to the disadvantage of requiring large quantities of water, the wet scrubbing processes generate a large amount of waste product solutions which are hard to dispose of without causing water pollution. Further, scaling or solidification of the reaction products occurs on various parts of the wet scrubbing system causing high maintenance costs.
In contrast to the wet scrubbing or washing processes, the prior art has also included a number of so called dry scrubbing processes of which the spray drying process is one type. The spray drying process, illustrated in FIG. 1, is superficially similar to the washing or wet scrubbing process in that water is used to inject an alkali reagent such as lime or such as lime or lime stone into the stream of affluent gases. However, unlike the washing or wet scrubbing process, the spray drying process uses a relatively small quantity of water which will evaporate after it has carried the reagent into a chamber through which the effluent gases pass. From the chamber, the effluent gases pass into a particulate collection means such as a fabric filter or electrostatic precipitator, whereat the solid products of the reaction between the reagent and the pollutant (sulfur dioxide) may be removed. As shown in the FIG. 1 block diagram of this prior art process, the purified effluent gases pass from the particulate collection means into the stack where they are discharged.
Although the spray drying process has been useful in removing pollutants without being subject to the disadvantages of the wet scrubbing or washing processes, the spray drying process has other disadvantages. Specifically, the spray drying process generally requires a higher stoichiometric ratio of reagent (usually calcium from lime or limestone) to sulfur oxides then is the case for the wet scrubbing processes. Typically, in order to achieve 90% removal for affluent gases from high sulfur content coal the stoichiometric ratio of reagent (calcium) content to sulfur oxides must be over 2:1. However, the solids content of the injected slurry in a spray drying process is limited to less than 30%, preferably under 25% to avoid damaging the pump which is used to inject the slurry into the spray dryer. Accordingly, one cannot raise the stoichiometric ratio of reagent to sulfur oxides unless the water content is increased. Yet the water content must be limited in order to keep the process dry and avoid the disadvantages of the washing processes discussed above. As a result of these conflicting considerations and as shown in curve A of FIG. 2, the spray drying process is generally limited to coals having less than 2% sulfur content. Curve A of FIG. 2 shows the relationship of solids content of the slurry to the sulfur content level in the coal as required to meet recent source performance standards for sulfur oxides removal set by the U.S. Environmental Protection Agency. It will thus be appreciated that the spray drying process is useful, but is somewhat limited in applicability because of the necessary design tradeoffs.
A third type of prior art pollution control system is that of ionizing radiation. Such systems use electron beam or ultraviolet light (gamma radiation and other types may as well used) to ionize the nitrogen oxides and sulfur dioxide in the affluent gases. Although the reaction mechanism for the oxidation of sulfur dioxide and nitrogen oxides using this technique is very complex and not fully understood, the ionization caused by the electron beam irradiation converts the sulfur dioxide and nitrogen oxides to acid mist at low temperatures and/or solid particles at high temperatures. An adaption of this process uses a preliminary desulfurizing method such as washing the effluent gases in a tower before radiating the gases.
Unfortunately, the electron beam method usually requires high dosages (two to eight megarads) to satisfactorily remove the pollutants. Additionally, the acid mist has the tendency to corrode the electron beam reaction chamber. An additional disadvantage is that the acid mist and/or solid particles from the electron beam reaction chamber require further processing before they can be disposed of.